Constant potential gradient dielectric heating device



July 25, 1950 J. E. JOY 2,

CONSTANT POTENTIAL GRADIENT DIELECTRIC HEATING DEVICE Filed Feb. 15, 1946 I INVENTOR Joseph E. Jog

cam

ATTORNEY IIH Patented July 25,1950

CONSTANT POTENTIAL GRADIENT DIELECTRIC HEATING DEVICE Joseph E. Joy, Collingswood, N. 1., assignor to Radio Corporation of America, a corporation of Delaware Application February 15, 1946, Serial No. 647,725

4 Claims.

This invention relates to the art of high frequency heatsealing, and in particular to a' method of and means for automatically compensating for the detuning effects caused by variations in thickness of thermoplastic materials being heat sealed between energized electrodes and for automatically'adjusting the radio frequency potential between the electrodes to apply the proper voltages to effect heat sealing for various thicknesses of material.

It is well known that there are a number of synthetic resins which become soft in the presence of suificiently high temperature and which may thus be bonded together by the application of heat. These materials are generally classed as thermoplastics, and include such commercial materials as Pliofilm, Vinylite, Koroseal and the like. These materials are very useful in the fabrication of waterproof coverings, air-tight enclosures and packages for the shipment and storage of foods, chemicals or other materials which deteriorate in the presence of air or moisture. It has been customary to seal materials of this type by passing the thermoplastics between heated rollers which bring the material'to the point of plasticity and permit bonding under pressure. The advantages of inducing radio frequency heat energy in thermoplastic materials held between bar or roller electrodes has been pointed out in an article by C. N. Hoyler in Electronics for August, 1943, entitled An Electronic Sewing Machine. He points out that when a radio frequency field is applied across the seam, heat is generated uniformly in the thermoplastic by dielectric loss. Due to the thermal conductivity of the electrodes which make contact with the outer surfaces of the thermoplastic, the softening effect is limited to the inner surfaces of the materials which are to be bonded. This makes possible a greatly improved bond without distortion of the outer surface of the material.

For a given thickness of thermoplastic, the maximum R. F; voltage between the electrodes which is permissible is determined by the characteristics of the material. If the voltage is too high, the material will break down and the radio frequency voltage will puncture the material causing faults in the bond. If the radio frequency voltage between the electrodes is too low there will be insufficient heating to cause a uniform bonding. It is therefore necessary to adjust the equipment carefully to produce the required voltage between the electrodes, taking into consideration the thicknessof the material,

2 its R. F. breakdown resistance and the speed with which it is desired to complete the bonding.

Commercial applications of radio frequency heating frequently require the bonding of seams in which a plurality of layers of materialpass between the electrodes at one time and relatively few layers of the material pass between the electrodes at other times. Due to the variation in the distance, and thus capacity, between the opposed electrodes, the normally resonant radio frequency circuit of which the electrodes are a part becomes detuned, thus radically changing the radio frequency voltage between the electrodes and causing an improper voltage to be impressed on the electrodes from time to time. It is expected that some commercial applications may require anywhere from two to eight layers of material to pass between the elec-' trodes in the process of sewing thermoplastic materials. It is the primary purpose'of this invention to provide an automatic means for adjusting the radio frequency voltage between the electrodes to maintain the proper value for all thickness of material. It is a further object" of this invention to provide an improved electronic sewing machine. A still further object of this invention is to provide means for automatically compensating for variations ofcapac ity between the electrodes of a radio frequency sewing machine caused by variations in the thickness of material being treated or in the number of layers of such material. I

This invention will be better understood from the following description when considered in connection with the accompanying drawings, and its scope is indicated by the appended claims;

In the drawings:

Figure 1 is a schematic view of one embodiment of this invention;

Figure 2 is a curve illustrating the operation of this invention; 1

Figure 3 is a plan view partly in cross section of a portion of an electronic sewing machine;

Figure 4 is a perspective view of a portion of an electronic sewing machine illustrating this invention; and

Figure 5 is an alternative embodimenushown in schematic form.

Similar elements in the drawings will be in-- dicated by similar reference numerals.

Referring to Fig. 1, thermoplastic materials to be bonded are passed between two roller electrodes l and 3. Electrode I is grounded, whilei electrode 3 is connected to the high potential: radio frequency source provided at the output sion 9 which is associated with and spaced from a grounded plate II, and constitutes a capacity therewith. Energy is supplied to electrodet :by

a wiping spring contact which is connected to the inner conductor of concentricline. 5.

It will be appreciated that as the thickness of material increases, it will be necessary to provide a higher voltage between electrodes 1 and 3 if the heating effect is to be maintained con stant. The required potential difference between the electrodes is approximately proportional to the th ckness of the material. that the p e tial gradient betweenthe two electrodesshould beapproximately Constant, In actual Practice there will be a small. deviation i'romthis theoretical value particularly when very thin layers of materiaLare being processed, since in that case arelatively large amount of the generated heat will pass to the coolelectrodes and thus be lost The total effect is, of course dependent upon the heat 'conductivityof the material in question. Whenthicker layers, or a largerhumber of layers, arebeingused, those layers not in direct contact with the electrodes do not tend to lose ;their heat-and therefore the heating is effected-more efiicientlyand a slightly lower po-,

tential. gradient will accomplish the desired result. To accomplishthe automatic adjustment of the-potentialgradient in accordance with the present; invention, ,1 take advantage of the change in capacit v between the two electrodes or between the high potential electrode and ground or a combination of both, occasioned by the movement of. one of the electrodes produced by variations inthickness of the material to be treated. The method of. accomplishingthis adjustment is. illustrated .bypthe curves shown in Fig. 2, to which reference is now made- The curves represent. the voltage vs. capacity resonance of the concentric line 5, including the capacitive loading produced by electrode '3 together with variable capacitor l3 .which may be connected across the high potentialterminal of the concentric line.

Tworesonant curves. are now shown, the one on the leftrepresenting the condition when the output circuit is substantially resonant at the operating frequency, while the other curve on the right shows the displacedcondition of resoname in which the operating frequency lies well down on the slope of the resonant curve. Movement between these two conditions will of course cause the output voltage tovary in the manner desired.

As is well'known, the voltage across the resonant circuit and thusbetween the electrodes 1 and 3, is a maximum at the point of resonance. The frequency of the oscillations produced by the radio frequency generator 1 is represented by the vertical line I which remains fixed at a value of the order of 200 .megacycles. Capacity I3 is then adjusted to tune the system to the low frequency side of. resonancawhen the thinnest material to be treated is between electrodes I and 3, by an amount sufiicient to bring theoperating point I! to a positioncn the rightv 4 hand resonance curve which will produce the desired potential difference between the electrodes. When a thicker material comes between the electrodes they will move apart so as to accommodate the increased thickness, and this will inherently reduce the capacity between the two. From Fig. 2 it will be seen that the point of operation then moves nearer to resonance and a higher potential difference appears between the two electrodes. The maximum thickness of material to be treated should likewise fall on the low frequency side of resonance, for example, at point 19 on the left hand curve. If the inherent capacity change between the two electrodes is nottsufiicient to produce the desired capacity change, additional capacity may be provided by means of plate H which is capacitively coupled to the extension 9 of the high potential electrode 3. It will be understood that the total capacity change may be adjusted to any value by suitable selection of the size of the capacity H or by adjustment of its spacing from electrode 9. The overall .rangjerof the capacity variation, and thustherange between maximum and minimum voltages, maybe controlled by suitably adjusting the ratio of capacity [3 with respect to the net capacity change eifect ed by the movement of the electrode. These values must be. determinedin'any. particular case by the specific requirements, and will'd'epe'nd on the operating "frequency, andthe nature of the material be- I ingtreated.

A practical embodiment of this invention as applied to a radio. frequency sewing machine. is

illustrated in Figs'..3 and 4, to-which reference is now made. .Fig., 4 isa perspective view of the'under side of the bed plate 2f which is also shown in cross sectional elevation inFig. .3. The.

quarter wave concentric line 5 is mounted under the bed plate 2i and. its outercon'ductor is ro nded .thereto. supplied fron a source, "not. shown, through concentric cable 23..v To provide anadjustable capacity as indicated by capacitor IS in Fig. 1, the inner conductor of the concentric line 5 is provided withan enlarged section at its outer end which may be brought into capacitive rela'.

tionship with a grounded ring, adjustably movable. in along'itudinalislot and'held by a set screw 25 An aperture Zl'I'is provided in the bed plate 2| throughwhich a portion of the highpo tential electrode 3 extends" so that itmay engage thematerial to be treated on the upper surface of the bed plate 2f. Electrode 3'consists of a metallic. ring supported by an insulating washer 29 which is in turn supported by a rotatable shaft 3L The'shaft is coupledto. adriving motor, not shown, through a gear 33 {and a pair of tim versal coupling eleme'ntsf35 and 31. whichpermit slight vertical movement. of the electrode at one end of the shaft without. affecting its rotation. At a point near the'electrode. the shaftfis supported by a bearing arm-3. which is pivoted at one end. in supports 4!" and 43' whichare fixed to thebed plate 2 l. A pin 45 extends at right angles from the bearing arm' 39. and is connected to a spring 41 which provides a .tensional force in such direction that the shaftt IT is resiliently held againstthe grounded electrode l whichis mount-v ed above. the bed plate. Asemi-cylindrical plate ll is mounted onthe bed plate 2 I, and capacitively coupled to the extension 9 ofthehighpo-v tential electrode 3. Radio; frequency energycis applied to the .electrodethrough abrush 49 Radio frequency voltage is conductor of concentric line 5 so as to engage the metallic outer portion of the electrode 3.

Cooperating with the high potential electrode 3 is a grounded electrode l which is mounted in the upper head portion 5| of the ewing machine. Electrode l is driven by the motor, not shown, which also drives gear 33, both electrodes rotating in such a direction and at such a speed that the material to be treated is caused to pass between them. There may alsobe provided a hand or foot operated mechanism for lifting electrode l upwardly so as to permit placement of the material to be treated between the two electrodes.

An alternative arrangement for automatically adjusting the capacity is, illustrated in Fig. 5, which shows schematically a grounded electrode I and a high potential electrode 3 positioned so as to contact opposed surfaces of the material 53 which is to be treated. Electrode 3 is energized by a connection to the inner conductor 55 of concentric line 5 and is mechanically coupled, as indicated byline 51, to a movable metallic ring 59 which cooperates with an enlarged section SI of the inner conductor 55'to form a loading capacity across the resonant circuit. The ring 59 may be located at either end of the enlarged section 6|. In one case its movement in a given direction will increase the loading capacity of the line and thus decrease its resonant frequency. In the other case, illustrated by the dotted line, movement in the same direction will decrease the loading capacity. Variations in the thickness of the material to be treated, which cause a change in the capacity between the electrodes, may thus control a capacity which is variable in such direction as to compensate for this change, or, as in the case previously discussed, to augment the change so as to maintain constant the potential gradient between the electrodes.

What I claim is:

l. A radio frequency device for heating a plurality of layers of thermoplastic materials of varying thicknesses comprising a pair of opposed electrodes adapted for engaging said layers of materials therebetween, said electrodes being rotatable to provide for movement of said materials in a predetermined direction, a resonant circuit including the capacity between said electrodes as an element thereof, means for resiliently positioning and biasing one of said electrodes for movement toward the other electrode to maintain a constant pressure on said materials when diiferent thicknesses of said materials pass between said electrodes, means including a concentric transmission line for applying radio frequency power at a predetermined substantially fixed frequency to said resonant circuit and for impressing a high frequency voltage between said electrodes, and means for adjusting the resonant frequency of said circuit substantially to and below said predetermined frequency when materials of maximum thickness to be treated are positioned between said electrodes, said last named means comprising a conductive extension element for the movable electrode, an auxiliary electrode element mounted in variably spaced relation to said extension element and providing therewith additional tuning capacity in said resonant circuit, and said extension element being movable with said movable electrode for decreasing the spaced relation with respect to said auxiliary electrode element in response to a reduction in thickness of said materials there- 2. A radio frequency heating device compris-' ing a pair of opposed electrodes adapted to receive therebetween various thicknesses of di'- electric material to be heated; resilient supporting and positioning means connected with said electrodes for bringing said electrodes into contact with opposite sides of said material so that the relative spacing of said electrodes is determined by the thickness of said material; a

source of radio frequency energy; conductor means including a resonant circuit for applying said energy to said electrodes; and variable capacity means comprising one of said electrodes and an auxiliary electrode element responsive to variations in the relative spacing of said electrodes for varying the resonant frequency of said circuit, and additional variable tuning means for adjusting the resonant frequency of said circuit substantially equal to and below the frequency of said energy when the thickest material to be heated is between said electrodes, and said first named variable capacity being operative in response to relative movement of the electrodes to lower said resonant frequency with respect to the frequency of said energy and to lower the applied voltage on said electrodes when the thinnest material to be heated is between said electrodes to a degree to maintain the potential gradient between said electrodes substantially constant.

3. A radio frequency heating device for uniting multiple layers of thermoplastic dielectric material, comprising in combination, a bed plate having an opening providing communication between the upper and lower sides thereof, a rotatable electrode positioned above said opening, a second rotatable electrode positioned below said opening within said bed plate and in opposed relation to said first named electrode, a resiliently mounted supporting shaft for said second electrode carried by said bed plate and extending along the lower surface thereof to carry said second electrode into engagement with said first electrode through said opening, concentric line means for supplying radio frequency power to said electrodes at a predetermined substantially fixed frequency and including a fixed tubular concentric line extending along the lower surface of said bed plate, a flexible contact element connected between the terminal end of said last named concentric line and said second electrode, means providing a conductive cylindrical extension for and coaxial with said second electrode, an adjustable auxiliary electrode carried by said bed plate in spaced relation to said electrode extension, the relative positions of said auxiliary electrode and extension electrode being such that the spacing therebetween increases in response to an increase in the thickness of material passing between said first and second electrodes, and a variable capacity device carried by said second concentric line providing jointly with said auxiliary and extension electrodes a tuning capacity for resonating said concentric line to a frequency of the order of and les than the power supply frequency when materials of maximum thickness are introduced between said first and second electrodes, and said auxiliary and extension electrodes being effective to increase said last named tuning capacity in response to a minimum thickness of material between said first and second electrodes and to shift the resonant of dielectric parts'of varying thickness; com prising a plurality of spaced; continuouslyro tating electrodes adaptedfor receivingand continuously advancing said parts therebetween, asource of high frequencyelectrical" energy; at least one of said electrodes beingyieldingly urged toward the other and separable from the other in response to the thickness of said parts, and

mean coupling'sai'd' electrodes to said source,

said means includingan inductance device and i a variable capacitor electrically connected in parallel with said electro'des'capable of varying the rate at which" energy is suppliedto said parts, said variable capacitor comprising a" generally cylindrical extension of the said yieldingly-urged electrode adapted to'rotate continuouslywith said electrode and" a generally curved plate cooperating with andspa'ced from *said cylindrical extension wherebyvariations in thickness of said parts in the region of said electrodes cause varia-' tions in the relative spacing of said cylindrical extension with respect to said curved plate so as to control said'rate.

JOSEPH E. JOY;

ama e 7 REFERENCES CITED The ionowmg' references are of record in the fi s-rear I UNITEDISTATESL PATENTS 7 Number Name Date 1,972,050 Davis Aug. 28,1934 2,127,408 Kaar' Aug. 16, 1938 2,147,689 Chaffee 1 Feb. 21, 1939 10 2,177,272 Zottu 1 Oct. 24, 1939 1 2,179,261 Keller Nov. '7, 1939 2,324,068 Crandell' -11 July 13; 1943 2,337,219 -Zottu Dec. 21, 1943 2,391,086 Crandell Dec. 18, 1945' 15 2,396,004" -Gilbert- Mar. 5, 19 46 2,432,412 I-Iacklander Dec.- 9,1947 2,434,330 Merz et a1. Jan. 13, 1948 FOREIGN PATENTS 20 Number Country Date 375,587 GreatBritain June 30, 1932 556,292 Great Britain Sept;28, 1943 OTHER REFERENCES Hoyler, An Electronic Sewing Machine,

Electronics, August. 1943,, page 91-93.

Zade, Welding. Thermoplastics .with High Frequencies, Plastics, September 1944, pages 30 and 32. 

